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6000 Years Ago

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6000 Years Ago

Six thousand years ago, northern Africa was a place of trees, grasslands, lakes and people. Today, it is the Sahara – a desolate area larger than Australia.

Lake Yoa, in northeastern Chad, has remained a lake through the millenniums and is still a lake today, surrounded by hot desert. Although little rain falls, Lake Yoa’s water is replenished from an underground aquifer.

By analyzing thousands of layers of sediment in a core, which is a column of sediment drilled from the lake bottom, a team of scientists has reconstructed the region’s climate as the savannah changed to the Sahara.

In the journal Science, the researchers, led by Stefan Kroepelin, a geologist with the Institute of Prehistoric Archaeology at the University of Cologne in Germany, report that the climate transition occurred gradually.

In particular, the changing types of pollen that fell on the water and drifted to the bottom tell a story of how the terrain shifted from trees to shrubs to grasses to sand – “where today you don’t find a single piece of grass,” Kroepelin said.

The findings run counter to a prevailing view that the change happened abruptly, within a few centuries, about 5,500 years ago, marking the end of the “African Humid Period” when monsoon rains poured down on the region.

That view arises from ocean sediment cores drilled off the coast, to the west of Mauritania. In 2000, analysis of the cores by researchers led by Peter B. deMenocal of Columbia University’s Lamont-Doherty Earth Observatory showed a sudden rise in the dust blown off Africa at that time.

Kroepelin did not dispute the ocean core data but said it had been “over-interpreted.”

Data about what was happening on land is sparse, because blowing sands do not preserve a clear geological record the way lake sediments do. But at Lake Yoa, the water that filled underground aquifers during the humid period, which began 14,800 years ago, is still flowing into the 60-foot-deep lake. It is enough to offset the 6 meters of water that evaporate out of the lake every year, Kroepelin said. Only a few millimeters of rain fall a year.

Kroepelin said he hoped to return to Lake Yoa next year to drill a core that could trace the climate history back 12,000 years.

DeMenocal praised Kroepelin’s research. “I think it’s a very good body of work,” he said. “It’s really the only thing of its kind from the arid interior.” But he said he wondered whether the pollen might have come mostly from the area immediately by the lake and not the larger Sahara.

Jonathan A. Holmes, director of the Environmental Change Research Center at University College London, said both sets of research had been carefully conducted, and the challenge would be to put together a more complex history of the area’s climate.

“I don’t think either record is somehow wrong,” said Holmes, who wrote a commentary accompanying the article in Science. “I think what they are representing are slightly different things.”

Holmes said one possibility was that the offshore dust might reflect a drop in water levels around Lake Chad, revealing more dust-producing soil.

However fast the drying occurred, it pushed people out of north-central Africa, deMenocal said, and that climatically forced migration might have led to the rise of the Pharaohs and Egyptian civilization.

Source: signonsandiego.com

Climate change ‘was the making of civilisation’

Written by aedh

October 23, 2009 at 6:26 am

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